This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-106580, filed Apr. 14, 1999; and No. 2000-001026, filed Jan. 6, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a near field optical microscope. More particularly this invention relates to a near field optical microscope using scattering probe and a method for observing samples using the near field optical microscope.
Further, this invention relates to a near field optical microscope. More particularly, this invention relates to a near field optical microscope using scattering probe comprising means for improving the S/N of a near field signal when the signal strength changes in accordance with changes in the wavelength of illumination light.
A scanning probe microscope (SPM) is a general term for devices which provide a probe at a field of less than 1 xcexcm to the surface of a sample, detect the interactive force between the probe and the sample while the probe scans in the XY direction or the XYZ direction, and perform two-dimensional mapping of this relative effect. SPMs of this type include, for example, a scanning tunneling microscope (STM), an atomic force microscope (AFM), a magnetic force microscope (MFM), and a scanning near field optical microscope (SNOM).
Of the above microscopes, the SNOM in particular has been developed since the late 1980s as an optical microscope which has a resolution exceeding the limits of diffraction, and which detects near field light provided close to a sample, for fluorescent light measurement of living body samples, evaluating measuring elements and materials for photonics (evaluation of various characteristics of dielectric photoconducting wave guides, light-generating spectral measurement of semiconductor quantum dots, evaluation of various characteristics of light-generating elements on the surface of a semiconductor, etc.), and the like.
Basically, the SNOM is a microscope which provides a sharp probe near to the sample under illumination light, and detects the state at the position (position of near field) where the light is near to the sample.
U.S. Pat. No. 5,272,300 (Literature 1) appended by Betzig et al. on Dec. 21, 1993, discloses an SNOM which injects light into a sharp-tipped probe, thereby generating a position of local light near a very small aperture in the tip of the probe, and illuminates a very small section of the sample by touching it with the tip of the probe. A light detector is provided below the sample, and detects the light which has permeated the sample, whereby the SNOM performs two-dimensional mapping of the strength of the permeated light.
This SNOM uses a sharp-tipped rod-like probe such as an optical fiber or glass rod, or a crystal probe.
An improved version of this rod-like probe, wherein the areas of the probe other than the tip are coated with a metallic film, is commercially available.
A microscope using this probe has better horizontal resolution than a microscope using a probe which is not coated with a metallic film.
In xe2x80x9cJ. Microscopy 177 (1995) p. 115xe2x80x9d (Literature 2), Heinzelmann et al. disclosed a method for achieving high resolution by providing a movable light detector, determining the scatter angle dependency of the signal, and using a signal of a specific angle.
The most common type of SPM is the AFM, used as a microscope which obtains information relating to the unevenness of the sample surface.
The AFM has a probe supported on the tip of a cantilever, and uses, for example, an optical displacement sensor to detect the displacement of the cantilever in correspondence with forces acting on the probe when the probe is positioned near the surface of the sample, thereby indirectly obtaining information relating to the unevenness of the sample surface.
For example, one such AFM is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 62-130302 (Literature 3).
This AFM uses a method for measuring the unevenness of the sample by detecting the interactive force between the sample and the probe which is also used in other SPM devices. The method is termed regulation, and comprises maintaining a fixed distance between the sample and the probe.
In xe2x80x9cAppl. Phys. Lett. 62(5) p. 461 (1993)xe2x80x9d (Literature 4), N. F. van Hulst et al. propose a new SNOM which detects optical information of a sample using a silicon nitride cantilever for AFM to measure the unevenness of the sample by AFM measurement.
In this microscope, the sample is provided on an internally all-reflecting prism, and He-Ne laser light is illuminated from all sides of the prism onto the sample, exciting the sample and forming a spot of evanescent light near the surface of the sample.
The probe supported at the tip of the cantilever is inserted into this spot of evanescent light, converting the local waves of evanescent light into propagated waves of scattered light. A portion of this light enters the probe, which comprises silicon nitride and is almost completely permeable by Hexe2x80x94Ne laser light, and escapes to the rear side of the cantilever.
This light is condensed by a lens, provided above the cantilever, and illuminated through a pinhole, provided at a position corresponding to the position of the probe with respect to the lens, into a photo multiplier tube which outputs an SNOM signal.
During the SNOM signal detection, an optical displacement detection sensor measures the displacement of the cantilever as in normal AFM measurement, and for example a piezoelectric scanner is controlled by feedback so that the displacement maintains a predetermined constant value.
Therefore, during one scanning process, SNOM measurement is carried out based on the scanning signal and the SNOM signal. In addition, AFM measurement is carried out based on the scanning signal and the feedback control signal.
In an aperture-type SNOM such as that disclosed by Betzig et al., the probe should preferably be coated with metal in order to achieve a high horizontal resolution.
However, it is not easy to uniformly mass-produce metal-coated probes having apertures in their tips. An SNOM demanding ultra high-resolution requires a resolution exceeding the resolution possible when using a normal optical microscope. To achieve this, the diameter of the aperture in the tip of the probe must be less than 0.1 xcexcm, and preferably less than 0.05 xcexcm.
It is extremely difficult to manufacture an aperture of such a small diameter with good reproducibility.
The quantity of light illuminated through the aperture into the probe decreases in proportion to the square of the radius of the aperture. Consequently, there is a problematic trade-off, since, when the diameter of the aperture is reduced with the aim of increasing the horizontal resolution of the SNOM image, the quantity of detected light decreases and the S/N ratio of the detection system worsens.
Accordingly, a new SNOM (dispersion mode SNOM) wherein no aperture is provided in the tip of the probe has been proposed. In this SNOM, a highly refractive dielectric having a structure less than the wavelength, or a metal, strongly scatters the near field light.
Since this SNOM does not require an aperture in the tip of the probe, the problem of difficulty in manufacturing the aperture, and the trade-off problem, do not arise.
Kawada et al. disclosed a scattering-type mode SNOM in Jpn. Pat. Appln. KOKAI Publication No. 6-137847 (Literature 5).
In this SNOM, a needle-like probe scatters evanescent light formed onto the surface of the sample, thereby converting it to propagated light. The propagated light, that is, the scattered light is detected by an objective lens and a light detector, which are provided at the side of the probe, and optical information relating to the sample is obtained based on the detected signal.
In xe2x80x9cThe 42nd Lecture Meeting of the Japanese Federation of Applied Physicsxe2x80x9d (Preliminary Papers No. 3, p. 916, March 1995) (Literature 6), Kawada et al. disclosed a device in which an STM metal probe is used as the probe. Propagated light is generated when evanescent light, formed on the surface of the sample, is scattered by the tip of the metal probe. The device performs STM observation and SNOM observation by observing this propagated light from the horizontal direction of the sample and the probe, while controlling the distance between the sample and the probe by STM.
Furthermore, in xe2x80x9cThe 43rd Lecture Meeting of the Japanese Federation of Applied Physicsxe2x80x9d (Preliminary Papers No. 3, p. 887, March 1996) (Literature 7), Kawata et al. announced that it is possible to perform SNOM observation without using evanescent light. Instead, propagated light is diagonally illuminated onto the sample from thereabove, and scattered in multiplex between the sample and the tip of a metal probe.
In xe2x80x9cOpt. Lett. 20 (1995) p. 1924 (Literature 8), Bachelot et al. announced a scattering-type SNOM which does not use an aperture probe, light being propagated from above.
In Jpn. Pat. Appln. KOKAI Publication No. 9-54099 (Literature 9), Toda et al. disclosed a scattering-type SNOM utilizing a suggested field illumination system using a microcantilever for AFM.
In these scattering-type SNOMs, in order to achieve an accurate observation result, it is most important to detect the scattered light efficiently, that is, to obtain an SNOM signal (a near field signal) having a high S/N.
Furthermore, a new SNOM (scattering-type SNOM) using a metal or a highly refractive dielectric having a structure less than the wavelength, to strongly scatter the near field light.
In xe2x80x9cPhys. Rev. Lett. 62 (1989) p. 458xe2x80x9d (Literature 10), Fischer et al. disclosed an SNOM in which an extremely small metal sphere on a transparent flat face is provided above the sample, and a laser approximately at a plasmon resonance frequency is illuminated from above. Consequently, local plasmon is generated on the metal sphere, and an image is output by using the local plasmon as scattered light.
The scattering efficiency from the tip of the probe is strongly dependent on the wavelength of the light, the material comprising the scattering body, and the size of the scattering body. It is known that the material and size of the scattering body, and the wavelength of the light can improve the scattering efficiency when the light scattered by the probe has generated plasmon resonance.
For example, when light is scattered using a scattering body comprising gold, and assuming that the gold particle resembles a dot, an investigation of the scattering efficiency when the wavelength is altered shows that the peak of 10 nm width where the light wavelength is close to 550 nm.
When light having a wavelength near the peak is used as the illumination light, the scattering efficiency increases, thereby obtaining an S/N higher than when light having other wavelengths is used.
As described above, various types of SNOM have been disclosed. However, they have a disadvantage of picking up scattered light from strong scattering sources other than the probe, and consequently the image S/N deteriorates.
Various ideas for improving the S/N in signal detection using a near field microscope have been proposed as solutions.
The following method is often used to overcome the above problem.
When the probe is vertically oscillated near the surface of the sample, multiplex scattering between the probe and the sample is occurred only when the probe has moved near to the sample. Consequently, the signal at this point oscillated synchronously with the probe oscillation.
By contrast, the scattered light from the scattering source remains constant over time.
A lock-in amplifier extracts the amplitude of a signal strength in synchronism with the number of oscillations of the probe from the signals received by the light detector. In addition, by restraining the strength of the light from the scattering source, it is possible to obtain a signal only from the probe.
Furthermore, in Jpn. Pat. Appln. KOKAI publication No. 10-170522 (Literature 11), Sasaki discloses an idea applying hetrodine in combination with the above method for detecting near field microscope signals.
In a scattering-type SNOM, solving the disadvantage of picking up scattered light from strong scattering sources other than the probe, whereby the image S/N deteriorates, is an extremely important second problem.
It is an object of this invention to provide a solve the first problem mentioned above by providing a scanning near field optical microscope which is capable of detecting near field signal with superior S/N, and a method for observing a sample using the near field optical microscope.
It is another object of this invention to solve the second problem mentioned above by providing a near field optical microscope comprising signal detection means which applies modulation to a signal from a signal by modulating the wavelength of the illumination light, and extracts a SNOM signal from the modulated signal, thereby obtaining an image having good S/N.
In order to solve the first problem mentioned above, a first aspect of this invention provides a near field optical microscope comprising:
a light illumination portion which illuminates light to the surface of a sample;
a probe having a tip smaller than the wavelength of light illuminated by the light illumination portion, the tip being provided near the sample surface to which the light is illuminated, and scattering the light;
a light detection portion which detects light scattered by the probe; and
a scanning portion which relatively scans the sample and the tip of the probe,
wherein plasmon resonance is generated during scattering by the probe.
In order to solve the first problem mentioned above, a second aspect of this invention provides a near field optical microscope comprising:
a light illumination portion which illuminates light to the surface of a sample;
a probe having a tip smaller than the wavelength of light illuminated by the light illumination portion, the tip being provided near the sample surface to which the light is illuminated, and scattering the light;
a light detection portion which detects light scattered by the probe; and
a scanning portion which relatively scans the sample and the tip of the probe,
wherein conditions comprising the wavelength of the light illuminated by the light illumination portion, the refractive index of the tip of the probe, and the size of the tip of the probe, are conditions which generate the plasmon resonance during scattering by the probe.
In order to solve the first problem mentioned above, a third aspect of this invention provides a method for observing a sample using a near field optical microscope comprising the steps of:
relatively scanning a sample, and a probe having a tip provided at a position near a surface of the sample;
illuminating light to the tip of the probe, and scattering the light; and
detecting scattered light which is scattered by the tip of the probe,
wherein plasmon resonance is generated during scattering by the probe.
In order to solve the second problem mentioned above, a fourth aspect of this invention provides a near field optical microscope comprising:
a light illumination portion which illuminates light to the surface of a sample;
a probe having a tip provided near to the sample, the tip of the probe generating scattered light originating in light illuminated by the light illumination portion;
a light detection portion which detects light scattered by the probe;
an illumination light modulation portion which modulates the wavelength of the light illuminated by the light illumination portion at a predetermined frequency; and
an extraction portion which extracts components at the predetermined frequency of the illumination light modulation portion from an output of the light detection portion.
In order to solve the second problem mentioned above, a fifth aspect of this invention provides a near field optical microscope comprising:
a light illumination portion which illuminates light to the surface of a sample;
a probe having a tip provided near to the sample, the tip of the probe generating scattered light originating in light illuminated by the light illumination portion;
a light detection portion which detects light scattered by the probe;
an illumination light modulation portion which modulates the wavelength of the light illuminated by the light illumination portion at a first frequency;
a probe oscillation portion which oscillates the probe at a second frequency; and
an extraction portion which extracts a beat frequency component of the first frequency of the illumination light modulation portion and the second frequency of the probe oscillation portion from an output of the light detection portion.
In order to solve the second problem mentioned above, a sixth aspect of this invention provides a near field optical microscope comprising:
a light illumination portion which illuminates light to the surface of a sample;
a probe having a tip provided near to the sample, the tip of the probe generating scattered light originating in light illuminated by the light illumination portion;
a light detection portion which detects light scattered by the probe;
an illumination light modulation portion which modulates the wavelength of the light illuminated by the light illumination portion at a first frequency;
a coherence portion which makes a reference light, at a frequency differing only by a frequency xcex4 from the scattered light generated by the probe, coherent with the scattered light; and
an extraction portion which extracts a beat frequency component of the first frequency of the illumination light modulation portion and the frequency xcex4 of the coherence portion from an output of the light detection portion.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.